Single mode optical fiber sensors are sensitive to the state of polarization of light in the fiber. If the fiber is not significantly polarization-holding or preserving, waves of different polarization may have different phase velocities. Thus, for single-mode transducers, it is desirable to use fibers having geometric birefringence or other kinds of polarization-holding fibers. See, e.g., McMahon et al., "Fiber-Optic Transducers," IEEE Spectrum, December 1981, pages 24-27.
Even in an optical fiber that has good polarization-holding properties, however, unwanted guided waves can be present. For example, in an optical fiber having a core with an elliptical cross-section, in which the desired wave is usually polarized along the major axis of the ellipse, an unwanted wave can be present and will be polarized along the minor axis of the ellipse. Such an unwanted wave can originate, for instance, from a laser source which launches imperfectly polarized light into the fiber or from coupling between polarizations in the fiber itself. Although such unwanted waves can be tolerated during transmission, they become extremely undesirable at the output end of the fiber where the phase of the desired wave is sensed and used for various purposes.
Dyott U.S. patent application Ser. No. 404,283, (abandoned) filed Aug. 2, 1982, entitled "Self-Aligning Optical Fiber With Accessible Guiding Region" and assigned to the assignee of the present invention, discloses a self-aligning optical fiber comprising a core and cladding having different refractive indices and forming a guiding region located close to the surface of the fiber along a selected length of the fiber. The core of the fiber has an elliptical cross-section, and the outer surface of the fiber has a non-circular (preferably D-shaped) cross-section with a predetermined geometric relationship to the elliptical core so that the orientation of the major and minor axes of the elliptical core can be ascertained from the geometry of the outer surface. The elliptical cross-section of the core in combination with the different refractive indices of the core and cladding permit the de-coupling of waves polarized along such axes.
While an optical fiber of the type disclosed in the Dyott application permits the de-coupling of waves polarized along two transverse orthogonal axes, a separate polarizing device or polarizer is required to suppress the waveguide's ability to guide one of the two orthogonally polarized modes (e.g., the .sub.e HE.sub.11 mode) and to cause the waveguide to guide only the other mode (e.g., the .sub.o HE.sub.11 mode) and thereby limit the waveguide to true single-mode operation.